Among xylene isomers, the most industrially important one is p-xylene. At present, p-xylene is used as a raw material for producing terephthalic acid, which is a monomer constituting polyesters that are ranked with nylons as major polymers. Its demand is high especially in Asia in recent years, and this trend is expected not to change in the future. On the other hand, since the demands of other xylene isomers, o-xylene and m-xylene, are much smaller than that of p-xylene, it is industrially important to convert o-xylene and m-xylene to p-xylene.
The raw material of p-xylene is a C8 aromatic hydrocarbon mixture. Since C8 aromatic hydrocarbon mixture generally contains high boiling components having not less than 9 carbon atoms in addition to xylene isomers and ethylbenzene, these high boiling components are first removed by distillation. The purified C8 aromatic hydrocarbon mixture is supplied to p-xylene-separating step to separate p-xylene. Since the boiling points of xylene isomers and ethylbenzene are close, it is difficult to separate p-xylene by distillation. Therefore crystallization or adsorptive separation is utilized.
In the case of crystallization, since a eutectic mixture of p-xylene, other xylene isomers and ethylbenzene is generated, the recovery of p-xylene per one path is limited, and is usually limited to about 60% at most. In case of crystallization, the higher the concentration of the p-xylene in the C8 aromatic hydrocarbon mixture supplied to the crystallization, not only the higher the productivity, but also the higher the recovery of p-xylene per one path.
In the case of adsorptive separation, almost 100% of p-xylene can be recovered in one path. In the adsorptive separation, the key component in the C8 aromatic hydrocarbons, which most strongly inhibits the separation of p-xylene, is ethylbenzene. Therefore, by decreasing the concentration of ethylbenzene in the C8 aromatic hydrocarbon mixture supplied to the adsorptive separation, since the load of the adsorptive separation can be decreased due to the decrease in ethylbenzene which is an obstacle to the separation, and since the p-xylene concentration in the C8 aromatic hydrocarbon mixture to be supplied to the adsorptive separation can be increased, the production capacity of p-xylene in the same adsorptive separation equipment can be increased.
The C8 aromatic hydrocarbons from the p-xylene-separation step, having a low concentration of p-xylene, are then transferred to a xylene-isomerization step and xylene isomers are isomerized with a zeolite catalyst to a p-xylene concentration close to that in the thermodynamic equilibrium composition. After removing the by-products having lower boiling points than the C8 aromatic hydrocarbons by distillation, the resulting product is mixed with the above-described fresh C8 aromatic hydrocarbon mixture and the resulting mixture is recycled to high boiling components separation step to remove by distillation the high boiling components having not less than 9 carbons, followed by separation and recovery of p-xylene again in the p-xylene separation step. This series of cycle is hereinafter referred to as “separation-isomerization cycle”.
FIG. 4 is a flow chart showing this “separation-isomerization cycle”. Usually, the C8 aromatic hydrocarbon mixture which is the raw material of p-xylene is transferred to a high boiling components separation step 1 from the supply line denoted by stream 36. In cases where it is desired to remove the low boiling components contained in the fresh C8 aromatic hydrocarbon mixture, the mixture is supplied to a low boiling components separation step 4 from the supply line denoted by stream 45. In some cases where it is not necessary to remove the high boiling components and the low boiling components, the fresh C8 aromatic hydrocarbon mixture is directly supplied to a p-xylene-separation step 2 from the supply line denoted by stream 46. In either case, the fresh C8 aromatic hydrocarbon mixture is transferred to the p-xylene-separation step 2 together with C8 aromatic hydrocarbon components isomerized to attain a p-xylene concentration close to that in the thermodynamic equilibrium composition in a xylene-isomerizing step 3. In the high boiling components separation step 1, the high boiling components are removed through a line denoted by stream 38. The C8 aromatic hydrocarbons from which the high boiling components have been removed are transferred to the p-xylene-separation step 2 through a line denoted by stream 37, and p-xylene is separated through the line denoted by stream 39. The C8 aromatic hydrocarbons having a low p-xylene concentration are transferred to the xylene-isomerizing step 3 through a line denoted by stream 40, and isomerized to attain a p-xylene concentration close to that in the thermodynamic equilibrium composition. To the xylene-isomerizing step, hydrogen or a hydrogen-containing gas is also transferred through a line denoted by stream 41. The C8 aromatic hydrocarbon mixture from the xylene-isomerizing step, which contains by-products, is transferred to a low boiling components separation step 4 through a line denoted by stream 42, and the low boiling components such as benzene and toluene generated as by-products in the xylene-isomerizing step are removed through the line denoted by stream 43. The p-xylene-enriched stream containing high boiling components is transferred to the high boiling components separation step 1 through the line denoted by stream 44. The p-xylene-enriched stream is again recycled to the p-xylene-separation step 2 after removing in the high boiling components separation step 1 the high boiling components generated as by-products in the xylene-isomerizing step.
As described above, the C8 aromatic hydrocarbon mixture supplied to the “separation-isomerization cycle” contain ethylbenzene. In the above-described “separation-isomerization cycle”, the ethylbenzene is not removed and remains in the cycle, so that ethylbenzene accumulates. If the ethylbenzene is removed in some way to prevent accumulation thereof, ethylbenzene in an amount corresponding to the degree of removal thereof circulates in the “separation-isomerization cycle”. If the amount of the circulating ethylbenzene is decreased, the total amount of the circulation is also decreased, so that the energy unit consumption is decreased, which is greatly advantageous from the economical viewpoint. In addition, since the p-xylene concentration is increased and the ethylbenzene concentration is decreased in the C8 aromatic hydrocarbon mixture to be supplied to the p-xylene-separation step, the load in the p-xylene-separation step can also be decreased, which leads to the increase in the production of p-xylene.
The methods for removing ethylbenzene usually employed include the method in which an ability to convert ethylbenzene is given to the isomerization catalyst used in the xylene-isomerizing step, thereby converting ethylbenzene to xylene or to a substance which can be easily separated from xylene, in the isomerization step, that is, the reforming method in which ethylbenzene is isomerized to xylene simultaneously with the isomerization of xylene in the isomerization step (for example, JP 49-46606 B); and the dealkylation method in which ethylbenzene is converted to benzene and ethane by hydrogenation and dealkylation thereof in the isomerization step of xylene, and then the benzene is separated by distillation in the subsequent distillation step (for example, JP 57-200319 A).
In the reforming method, since it is necessary to give to the catalyst hydrogenation/dehydrogenation ability, it is indispensable that the catalyst contain platinum which is a very expensive noble metal. Further, to convert ethylbenzene to xylene, the reaction mechanism requires mediating the reaction through a non-aromatic hydrocarbon such as naphthene or paraffin, and the non-aromatic hydrocarbon exists in the product at a concentration from several percent to ten and several percent, and circulates in the “separation-isomerization cycle”. Further, since the ethylbenzene conversion rate in the reforming method is restricted by the thermodynamic equilibrium, the conversion rate is only about 20% to 50%.
On the other hand, in the dealkylation method, since only the hydrogenation ability to hydrogenate the ethylene generated by dealkylation of ethylbenzene is need to be given to the catalyst, a hydrogenation-active metal which is less expensive than platinum may be used, or even when platinum is used, the content thereof can be largely reduced, so that the catalyst is inexpensive. Further, since the reaction between ethylene and hydrogen is very quick, which ethylene is generated by the dealkylation reaction of ethylbenzene, the dealkylation reaction of ethylbenzene proceeds as if it is a substantially non-equilibrium reaction, and a very high ethylbenzene conversion rate can be attained.
Under these circumstances, the dealkylation method in which the catalyst is inexpensive and the amount of circulating substances in the “separation-isomerization cycle” can be made smaller is mainly used.
In converting the ethylbenzene in ethylbenzene-containing C8 aromatic hydrocarbons to benzene by dealkylation and in isomerizing o-xylene and m-xylene to p-xylene, (1) to make the ethylbenzene conversion rate as high as possible is preferred to decrease the energy unit consumption for the production of p-xylene so as to increase the production of p-xylene; (2) to make the conversion rate to p-xylene as high as possible is preferred to increase the p-xylene concentration in the C8 aromatic hydrocarbons circulating in the “separation-isomerization cycle” so as to promote the productivity of p-xylene; and (3) to make the xylene loss as small as possible is preferred to decrease the raw material unit consumption in the p-xylene production so as to decrease the production cost of p-xylene.
On the other hand, the usually used raw material of p-xylene is the C8 aromatic hydrocarbon mixture which is the reformate obtained by reforming naphtha and subsequent fractional distillation. A representative composition of this C8 aromatic hydrocarbon mixture is as follows: ethylbenzene: 18% by weight, p-xylene: 19% by weight, m-xylene: 42% by weight, and o-xylene: 21% by weight. However, with the increase in the demand of p-xylene, supply of the above-described reformate C8 aromatic hydrocarbon mixture tends to be short. Further, under the circumstances where it is emphasized that the amount of petroleum resources in the world is limited and petroleum will deplete in some day, C8 aromatic hydrocarbon mixture generated from thermal cracking, hereinafter referred to as “pyrolysis gasoline”, is now attracting attention. A representative composition of such “pyrolysis gasoline” is as follows: ethylbenzene: 60% by weight, p-xylene: 8% by weight, m-xylene: 19% by weight, and o-xylene: 10% by weight, non-aromatic components: 3% by weight.
When compared with the reformate C8 aromatic hydrocarbon mixture, since “pyrolysis gasoline” has a higher ethylbenzene concentration, ethylbenzene accumulates in the “separation-isomerization cycle” and the amount of the ethylbenzene circulating in the “separation-isomerization cycle” is increased, so that the load in the p-xylene-separation step is increased, which leads to decrease in the p-xylene production, only a limited amount thereof has been used so far. Further, “pyrolysis gasoline” much contains not only ethylbenzene, but also non-aromatic hydrocarbons. Thus, when the dealkylation method is used, because of the large amount of the non-aromatic hydrocarbons circulating in the “separation-isomerization cycle”, the xylene loss in the xylene-isomerizing step is sharply increased and the deactivation rate of the catalyst is increased, which are problematic.
In view of these circumstances, in the conversion of ethylbenzene to benzene and ethane by the dealkylation method, the following four points are industrially important tasks for attaining increase in the production of p-xylene, decrease in the raw material unit consumption and energy unit consumption, and attaining stable supply of the raw material:
(1) A high ethylbenzene conversion rate can be attained.
(2) The feedstock containing non-aromatic hydrocarbons can be treated without increasing the deactivation rate of the catalyst.
(3) Xylene loss can be made small even if the ethylbenzene conversion rate is made high.
(4) A high conversion rate to p-xylene can be attained.
As a method by which xylene loss is small even if the ethylbenzene conversion rate is made high in the conversion of ethylbenzene to benzene and ethane by the dealkylation reaction, a method wherein a zeolite having a crystal size larger than 1 μm is used to decrease the diffusion rate of o-xylene (e.g., JP 8-16074 B) has been tried.
However, even if such a method is used, with a raw material such as “pyrolysis gasoline”, having a high ethylbenzene concentration and containing non-aromatic hydrocarbons in a large amount, the xylene loss is sharply increased.
In the isomerization reaction of xylene, if the reaction pressure is increased, bimolecular reaction such as the disproportionation reaction and transalkylation reaction, and aromatics ring hydrogenation, preferentially occur, so that xylene loss and generation of non-aromatic hydrocarbons are increased. Further, if a catalyst containing platinum is used as the hydrogenation/dehydrogenation component, the price of the catalyst is high. In addition, since the hydrogenation reaction of the aromatic hydrocarbons drastically proceeds due to the raise of the reaction pressure and reaction temperature, not only the xylene loss is increased, but also the recovery of the aromatic hydrocarbons is decreased (e.g., U.S. Pat. No. 4,899,001 B (Table 1)), which are problematic.

The methods for further decreasing the amount of the circulating ethylbenzene in the “separation-isomerization cycle” include a method wherein the ethylbenzene in the C8 aromatic hydrocarbon mixture is treated by the above-described dealkylation method to convert the ethylbenzene mainly to benzene and the generated benzene is separated by distillation before feeding the C8 aromatic hydrocarbon mixture to the “separation-isomerization cycle”, thereby largely decreasing the circulation of the ethylbenzene in the “separation-isomerization cycle” (e.g., JP 5-87054 B); and a method wherein the C8 aromatic hydrocarbon mixture is supplied to a xylene-isomerizing step having an ability of hydrogenation and dealkylation, and then the product is supplied to a p-xylene-separating step (e.g., JP 5-24661 A).
These methods are similar to the isomerization reaction in the “separation-isomerization cycle” in the respect that the reaction to convert ethylbenzene to benzene and ethane by dealkylation reaction is carried out. However, in cases where the former method is used, since the feedstock is not diluted with the C8 aromatic hydrocarbons circulating in the “separation-isomerization cycle”, especially in cases where “pyrolysis gasoline” containing large amount of non-aromatic hydrocarbons is used as the feedstock, the xylene loss is extremely increased and the deactivation rate of the catalyst is drastically increased. In cases where the latter method is used, although the feedstock is diluted with the C8 aromatic hydrocarbons circulating in the “separation-isomerization cycle”, since the amount of the feedstock of xylene-isomerization step is increased, even if the xylene loss is slightly increased, its influence on the raw material unit consumption is large, which is problematic. Thus, in cases where “pyrolysis gasoline” containing a large amount of non-aromatic hydrocarbons is used, if the prior art technique is applied, the deactivation rate of the catalyst is sharply increased and the xylene loss is also increased, so that the catalyst life is shortened and the raw material unit consumption is largely aggravated, which are problematic. Therefore, when “pyrolysis gasoline” containing a large amount of ethylbenzene is used and so the amount of the ethylbenzene circulating in the “separation-isomerization cycle” is desired to be decreased, these processes cannot be employed.
It could therefore be helpful to provide a process for converting ethylbenzene in a C8 aromatic hydrocarbon mixture containing a large amount of non-aromatic hydrocarbons to mainly benzene, by which xylene loss is small, the deactivation rate of the catalyst can be reduced, and a high conversion rate to p-xylene can be attained.
It could also be helpful to provide a process for producing p-xylene, by which the concentration of ethylbenzene in the C8 aromatics hydrocarbon mixture for p-xylene-separation step can be largely decreased.